1. Field of the Invention
The present invention relates to an optical disk, and more particularly, to a high-density readable only optical disk with a super resolution near field structure, which can read marks with a size smaller than the resolution of a laser beam.
2. Description of the Related Art
Because optical disks have a very small recording area per recording unit in comparison with existing magnetic recording media, optical disks have been widely used as high-density recording media. According to their characteristics, optical disks are classified into a Read Only Memory (ROM) type, a Write Once Read Many (WORM) type, and an erasable type. The ROM type of optical disks can only read pre-recorded data, the WORM type of optical disks allows a user to write data onto a disk only once, and the erasable type of optical disks allows a user to erase and re-record data.
One example of the WORM optical disks is a Compact Disk Recordable (CD-R) disk. In a CD-R disk, when a recording laser beam with a wavelength of 780 nm is focused on a recording layer made from an organic dye material such as cyanine and phthalocyanine, bonds in the organic dye material are broken. At the same time, the structures of a substrate and a reflecting layer are optically changed. Under this process, data are recorded on a CD-R disk. The recorded data are read with a low power of 1 mW or less. A CD-R disk has a storage capacity of about 650 MB, and thus is widely used for recording and reading various types of data, such as audio and video data.
However, the storage capacity of optical recording media such, as CD-R or CD-Rewritable (CD-RW), which require a recording wavelength of 780 nm that is insufficient to record dynamic image data. Therefore, such optical recording media with a small storage capacity are impractical in today's complex multimedia environment.
In order to solve this problem, Digital Versatile Disks (DVDs), which require a shorter laser wavelength of 630-680 nm have been developed. DVDs have a storage capacity of between 2.7 and 4.7 GB (on one side). Generally, DVDs can be classified into a DVD-read only memory (DVD-ROM) type, a DVD-recordable (DVD-R) type, a DVD-random access memory (DVD-RAM) type, and a DVD-rewritable (DVD-RW) type. Recording on the DVD-R disks is accomplished through the use of a recording layer in which bonds in an organic dye material are broken upon exposure to a recording laser beam. As for the DVD-RAM and the DVD-RW disks, a recording layer's phase change alters the optical characteristics of disks, and thus, data are recorded on or erased from disks. In particular, the DVD-R disks using an organic dye material are compatible with DVD-ROM, cost effective, and have a large storage capacity in comparison with other recording media. For these reasons, much attention has been paid to the DVD-R disks at present.
As can be seen from the above description, one of the most important issues in many of the optical media is to increase the storage capacity. In this regard, various attempts have been made to increase the storage capacity. The storage capacity of optical disks mainly depends on the number of tiny pits on a predetermined surface of optical disks and the characteristics of a laser beam used to correctly read the pits. Generally, light output from a laser diode spreads in the form of a beam with a definite width due to diffraction in spite of using an optical pickup's objective lens. This beam size is called “diffraction limit.”
General optical disks have a reading resolution limit, given by the expression: λ/4NA, where λ is the wavelength of a light source and NA is the numerical aperture of an objective lens. As can be seen from the above expression, the storage capacity of optical disks can be increased by shortening a light source's wavelength or using an objective lens with a larger NA. However, there are limitations in that recent laser technologies cannot provide such a shorter wavelength laser and an objective lens with lager NA is expensive. Furthermore, as the NA of an objective lens increases, a distance (working distance) between a pickup and a disk is remarkably shortened. Thus, the pickup and the disk might collide with each other. As a result, damage to the surface of the disk may be caused, resulting in data loss.
In order to overcome the reading resolution limit, optical disks having a super resolution near field structure (super-RENS) have been studied. In optical disks having such a super-RENS, a mask layer made of silver oxide is mainly used. FIG. 1 schematically shows an optical disk with a silver oxide mask layer. As shown in FIG. 1, the optical disk includes a transparent substrate 10, dielectric layers 14, mask layers 11, a recording layer 12 having marks 15, and a reflective layer 13.
In optical disks with the structure of FIG. 1, silver oxide is dissociated into silver particles and oxygen for data recording. On the other hand, for data reading, plasmons are formed on the surfaces of the silver particles. Such surface plasmons induce near field reading (NFR). Therefore, very small marks 15 beyond the diffraction limit can be read.
A mask layer 11 made of metal oxide such as silver oxide can be used in WORM optical recording media in which silver oxide is dissociated into silver particles and oxygen for data recording. This means that metal particles inducing a super resolution effect are generated during a recording process. For this reason, there is a problem in that a silver oxide mask layer cannot be used in ROM optical disks that store data in the form of pits on their substrates instead of a recording process.